Identification of sharp water-depth boundaries at +/-60 degrees latitudes on Mars using the Mars Odyssey Neutron Spectrometer

It has long been predicted that the transition zone from ice-free lower latitudes to ice-stable high latitudes is narrow. This transition zone has been observed using measurements of epithermal neutron currents by the Mars Odyssey Neutron Spectrometer (MONS). These boundaries correspond to a sharp increase in the depth of the stable water ice table from the surface (near both polar caps) to depths much larger than 1 m, where it cannot be detected from orbit using neutron spectroscopy. The depth below the surface of this ice table can be derived from thermal and epithermal neutron currents measured using MONS assuming a two-layer model. The macroscopic absorption cross section, given by the ratio of fast to thermal neutron currents, is a good proxy for this depth poleward of plus and minus 60 degrees latitude. Maps of both this cross section and the depth determined from thermal and epithermal neutrons reveal two latitudinal bands of maximum apparent depth at plus and minus 60 degrees that have FWHM widths of about 25 degrees, which corresponds to a physical boundary less than 1500 km. We interpret these bands to reflect a transition from water molecules held in hydrous minerals at lower latitudes to water in the forms of water ice and water of hydration at higher latitudes.